Whenever a new water main is laid, for example to service a new housing development, it must be cleaned out and sterilised prior to connection to the existing mains, and also pressure-tested. The procedures involved are governed by stringent regulations.
In the United Kingdom the responsible authority is the Drinking Water Inspectorate (DWI). All methods currently approved by the DWI require two separate visits to site to carry out pipe fitting operations. DWI requires the positive isolation of new mains until they have successfully passed a bacteriological test after sterilisation.
One known method of satisfying DWI regulations is illustrated in FIG. 1. In this method, a flanged T-piece connector 1 is installed in the existing main 2. The side arm of the T-piece 1 is connected to a sluice valve 3. Temporary works comprising a further flanged T-piece 4 and a flanged spigot 5 are installed between the sluice valve 3 and the new main 6. To achieve the desired isolation, however, a steel plate 7 (known as a “spade”) is interposed between the sluice valve 3 and the new main 6. The side arm of the second T-piece 4 is connected via a sluice valve 8 to a fire hydrant 9. The fire hydrant 9 is connected via a temporary by-pass-pipe 10 to a tapping 11 on the existing main 2. The by-pass pipe 10 is fitted with a double non-return check valve 12, an injection point 13 for sterilant and a meter 14.
The arrangement shown in FIG. 1 suffers from numerous disadvantages. First, the formation of the mains tapping 11 on the existing main 2 is problematic. It involves working on the main 2 while the main is under pressure and this carries a certain safety risk. If any problems occur when the tapping 11 is being made, the necessary remedial action may involve considerable interruption to the supply carried by the existing main 2. Secondly, after the new main 6 has been successfully connected to the existing supply, the temporary works (second T-piece 4, fire hydrant 9 etc) are generally discarded. This is wasteful and expensive.
Another standard method of commissioning a new main is illustrated in FIG. 2. In this case, the T-piece 21 installed in the existing main 22 is connected via a sluice valve 28 to a first fire hydrant 23, and the end of the new main 24 is connected to a second fire hydrant 25. The two hydrants 23,25 are connected by a temporary by-pass pipe 26, as in the first method. The two hydrants 23,25 are braced apart by telescopic struts 27. While this method does not involve any live operations on the existing main 22, it still involves the installation of temporary works which are generally subsequently discarded.
The most fundamental problem with both the methods described above, however, and with variations on those methods that are also commonly used, is that connection of the new main to the existing supply after sterilisation necessarily involves removal of at least some of the temporary components so that the steel spade (first method) or one or both hydrants (second method) can be removed. Thus, after the new main has been sterilised it is once again exposed to the environment, leading to a risk of chemical or bacteriological contamination. It is precisely that risk that the regulations governing the procedure are intended to eliminate.